Good point. Leaving aside issues of "What constitutes a word", this process seems like it would boil down to:

sub lcs {
    my ($string, $find) = @_;

    $string = join ' ', split ' ', $string;
    my @f = split ' ', $find;

    my ($starting_point, $max_length, $substr) = (0,0, '');

    foreach my $start ( 0 .. $#f - 1 ) {
        foreach my $len ( 1 .. $#f - $start - 1 ) {
            my $search = join ' ', @f[$start .. $start + $len ];
            if ( $string =~ $search ) {
                if ( $len > $max_length ) {
                    $max_length = $len;
                    $start_point = $start;
                    $substr = $search;
                }
            }
        }
    }

    return $search;
}
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